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texastwofour

I bought my 1979 924 NA about a month ago and have been slowly gathering pieces for the engine rebuild I plan to finish over a period of time. One inportant thing I still need to get is a stock engine so I can leave my runner in the car while I do this.
Basically, I'm looking to sqeeze more power out of this car without blowing into it. I'm kind of an old-school sportscar kind of guy. . .if I could cut the top off and put on wire wheels, I might.
I've got euro pistons for it and just picked up a nice set of Weber's.
I will probably ultimately open the head a little (big valve head from emw or the like), but will probably do that down the road.
It needs to be mentioned, however, because I'd like to increase the CR as much as I can and I'm sure this will need to be part of the equation.
I know the big valve head and the euro pistons will interfere so I guess that is not something I'll worry about too much. Custom pistons might reduce the interference but I've got other things like children to spend money on.
Is there an opportunity to shave the block here? how much? How high can I take the compression ratio? Again, I'll never turbo this car.
Webers should enable me to increase the compression quite a bit over cis, correct?
I was thinking of trying to get in the 10:1 range with the big valve head, euro pistons, and the webers (and free exhaust of course).
Comments?
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TX24
'79 924 NA
WIP!

924RACR · Posted: Wed Sep 02, 2009 9:35 pm Post subject:

Cam seems missing off the list here. Yes, higher CR will help a bunch - we run the ITB cars at 9.5:1, does just fine on 93 octane with tons of advance - you could probably swing 10:1.

If you want to deck the block (not shave the head), then forget the pistons - or buy pistons and leave the block alone. No reason to do both except burn money.

Lighten the rods and balance the rotating assembly. Makes a difference. Turn the flywheel too, take some weight off.

Don't go overboard on the headwork (porting and polishing) - easier to screw it up than make improvements here.

Consider a crank scraper too, from crank-scrapers.com. We use them in the ITB cars, pretty cheap and free power. Good oil control too.

Consider fresh valves with a good valve job. At very least, make sure they're freshly lapped-in.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

texastwofour · Posted: Thu Sep 03, 2009 1:29 am Post subject:

Right, I forgot to mention a hot street grind from Integral Cams.

924RACR · Posted: Thu Sep 03, 2009 2:19 am Post subject:

No, I mean porting and polishing. Bigger valves won't hurt.

You can't mill the head and get any CR - all the combustion chamber is in the piston/block, the head is already flat. But yes, you can deck the block, and that is the appropriate action (beyond fitting the euro pistons).

Search on the 931 HG - I think there was some discussion that maybe it worked out to the same thickness?
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

NA head gaskets were long ago superceded to the 931 part, they are now identical. Not sure if there ever was a different in thickness, but it's a moot point unless you are able to find an old stash somewhere.

OTOH, you could see about getting a custom Cometic gasket and specify the thickness you want to help achieve your CR goals. The only issue there is that I don't think they're willing to do a one-off piece, they've always required a run of at least 50. When we did the group buy earlier this year, we had 64 gaskets done, and I believe they have at least one E-tailer who also ordered some up, as they are listed as in stock on their site (horsepowerfreaks.com IIRC). But the ones on that site will all be the standard .051" variety.

Be aware that decking your block can have consequences for your timing belt tension roller. There is not alot of play available in the tension roller, and if you take too much off of the block, you'll have difficulty getting the belt tensioned, and it may also require you to invest in an adjustable cam sprocket to properly dial in the timing.

My $.02 is to not mess around with deck height. Run your Euro pistons and invest your money in the big valve head. The head is where most of the restriction is anyway. It's costly, yes, but in the long run, it's probably the best approach to freeing up the most power out of the NA. If you didn't have the Euro pistons already, I would suggest just getting custom pistons done up (may not be as expensive as you think), but it sounds like you already have a sunk investment.

The other cheap path to add'l power that no one talks about much around here is nitrous. What if you did your Euro pistons with a big valve head, and then gave it a 75-shot? On a fresh build, you should have little worries, and that would probably be your biggest bang for the buck (vs. custom pistons, dealing with the ramifications of decking the block, stroker kit, etc.). Just a thought.
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erstwhile owner of just about every 924 variant ever made

texastwofour · Posted: Sat Sep 05, 2009 2:36 pm Post subject:

I don't have that much in the euro pistons in the grand scheme of things.
If I went with custom pistons I'd probably hold onto those as well as the CIS in case I ever wanted to revert (or get another 4 or 5 924's as some of you seem to do)

I'll probably leave the block as-is and look to get a little more compression from some custom pistons.
I assume if I'm going with the big valve head I'll need to have a conversation with Jorge about the pistons for valve-relief cutouts so they don't interfere (or maybe interference would be unavoidable with pistons for more CR?)

Maybe I'll start with the euro pistons and the carbs and maybe take a little off the flywheel, then jump up to custom pistons and the big valve head and maybe lighter rods if I feel like spending the money on this particular car later.

I thought I read somewhere that the pistons can be swapped out without removing the block (but that it is a total PITA?)
If I could just drop in the euro pistons for now that would be great. I'd obviously lift the block and have it done for the custom pistons.

Or, and this is the big question, if I'm going to all of this trouble/work should I just bite the bullet and go for the stronger (more involved and more expensive) build now without looking back?

Nitrous is an interesting option. I don't need a drag car and really just want this car to be fun to drive on hill country roads and the occassional autox but having a little something extra every once in a while might be fun.

Thanks for the feedback. If anyone has any experince with custom pistons with the big valve head, feel free to chime in.
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TX24
'79 924 NA
WIP!

924-76 · Joined: 05 Jun 2006 Posts: 1489 Location: Ontario, Canada

How much more power can be expected from the Weber vs. CIS?
I read somewhere that the CIS is good up to 180 hp???

If the Weber are going to free up some hp I’m all for it.

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1976 924 N/A ROW SOLD
1980 931S
1981 931 Part car RIP

texastwofour · Posted: Sun Sep 06, 2009 4:09 pm Post subject:

My understanding , and this is all theory on my part at this point as I haven't yet started tearing it apart is that CIS might be good for up to 180 hp or so but the only tested way to get there is through forced induction. My thinking is that the carbs will be better for more compression as I will keep tis car na. My horsepower goals are a little more modest, maybe in the 150 range if I'm lucky, and the drivetrain stock should heve no problem with this modest power goal. This car has a good start with some suspension improvements.
The car runs Ok now if a little underpowered. After engine improvements (or maybe during) I might try to get rid of the dogleg and ultimately go to 5 lug with disc front and rear, I believe these are major concerns when trying to take thse cars over 150 hp or so, and have no real drawbacks. I don't expect the carbs to provide much hp unless coupled with the euro or custompistions for higher compression and the big valve head.
Anyone willing to bite on the custompiston route while maintaining a non-interference motor?
Also, sound is important to me and the carbs may help the throttle response? I totally agree with Ideola that the subsystems must all be contempleted when planning upgrades and I will continue to gather as much info as I can before I go about yanking my working stock engine. With the suspension this car is already fun to drive if a little needy in the appearance area.
I also plan to keep my stock engine, my CIS, etc. in case I blow up the project engine trying to reach higher compression.
I understand why most of the senior posters own multiples. Theres always a chance something terrible will happen. Any more advice form you guys about really increasing the power while staying NA is always welcome. Keep 'em coming
Thanks
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TX24
'79 924 NA
WIP!

texastwofour · Posted: Sun Sep 06, 2009 4:20 pm Post subject:

In addition to upgrading the engine, I also intend to achieve a lttle better performace by lightening this car somewhat so feedback on the best way to do this is also appreciated. I don't need a rear seat so this will be removed along with no spare, no tools, less carpet/ sound deadening, etc. in the back end.
I would obviously like to maintain the cars decent 51/49 weight ratio so 'd like to remove equivalent weight from the front off the car. I don't have my webers yet, nor have I removed the CIS. Will there be any weight savings in the front with carbs over CIS? What else is recommended to decrease the weight in the front of the car?
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TX24
'79 924 NA
WIP!

924-76 · Joined: 05 Jun 2006 Posts: 1489 Location: Ontario, Canada

-http://www.924board.org/viewtopic.php?t=29868
-fiberglass hood
-GTS headlight, look in the "how to" section
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1976 924 N/A ROW SOLD
1980 931S
1981 931 Part car RIP

texastwofour · Posted: Mon Sep 07, 2009 2:09 am Post subject:

Thanks for the tip, that is a great option. Since i'm getting the gear reduction starter and am all manual as well, a lighter weight battery can easily be done. Temperature is a non-issue in Texas. I just moved here a couple of years ago and but lived in Minnesota for 8 years before that. I'll never go back to snow, ice, and subzero temps (except to ski!) My "winter build" will be more like a "fall build" to you Northerners

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TX24
'79 924 NA
WIP!

Jakkq · Joined: 29 Sep 2008 Posts: 810 Location: Omaha, Nebraska

I wish it didn't snow here.

I'm goign to have to get ahold of a witner car.
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1979 Porsche 924- Snailshell
http://porsche924workblog.blogspot.com/

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

924RACR · Posted: Thu Sep 17, 2009 1:55 am Post subject:

Only problem is ground clearance... which is only a problem when you have a lot of snow... like in Nebraska...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Rocco R16V · Joined: 03 May 2009 Posts: 497 Location: PNW

If you removed a very well tuned CIS and put on webbers tuned to thier max on the same exact motor with same timing and same A/F ratio, the Webbers might gain you >5hp. Probably only one or two. this is due to the air restriction from the CIS air metering plate. but the webbers will have better throttle response, which will make a more a difference in feel than the HP gain.

There might be someone who builds and knows these heads better than me, (I have never built a porsche engine) but i would think you should (could?) go to 11/1 compression. This is about the max you can go without needing to use race gas.

There are people who have 8V VW's that run 13.5/1 on race gas

, 11/1 on 91oct, and I dont think those heads are as well designed as these are. so 10.5/1 should be easy and would be as low as i would want for N/A.
it will become a interference motor but the power will be worth it. your building the motor, will be changing the timing belt and doing maint on schedule so that shouldnt be an issue. There are real gains to be had by upping the compression.
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